Images of the distribution of radioactive materials in patients is customarily performed with radioisotope cameras as parallel ray projections. The collimation of the gamma rays emitted by the radioactive materials is provided by the radioisotope camera through parallel hole collimators or pinhole collimators. Additional information concerning the depth distribution of radioactive material, i.e., tomographic information, may be obtained by imaging with a radioisotope camera from multiple directions and mathematically estimating the tomographic distribution of radioactive materials.
Multiple projection information for tomographic imaging can be obtained by rotating camera systems, circular multidetector array systems, or stationary camera limited-viewing-angle-tomography systems. Rotating camera systems and reconstruction software are available in the commercial market. Multi-detector circular arrays are most commonly used for positron emission imaging and such systems are available commercially. Rotating camera systems require hardware, not normally found in conventional systems, which adds significantly to the cost of a radioisotope camera and may interfere with the normal use of the camera. Circular multidetector arrays are generally too expensive and are not suitable for conventional planar imaging.
Limited viewing angle tomography systems which can be implemented on a stationary radioisotope camera have been of interest because of the low cost associated with this approach. Two approaches are in common use and are commercially available; an array of 7 pinholes which generates separate images in a seven segment field of a radioisotope camera is described by Vogel, et al., "A New Method of Multiplanar Emission Tomography Using a Seven Pinhole Collimator and a Anger Scintillation Camera", J. Nucl. Med. 19:648-654, 1978 and a rotating slant hole collimator system which collects multiple views by rotating the collimator system as taught in U.S. Pat. No. 4,302,675 to Wake, et al. Both approaches provide projection information in only a limited number of directions.
Three other approaches have been described in the literature but have not been implemented commercially; the stationary non-redundant coded aperture, the fourier multi-aperture collimator and the time modulated coded aperture.
Stationary-coded techniques are attractive because of their potential for dynamic cardiac studies. However, reconstructions are subject to structured noise reflecting the cross correlation pattern of the code, and it is not possible to uniquely identify individual radiation ray paths and thus apply attenuation corrections. The inability to provide for attenuation corrections is a serious disadvantage.
The fourier multi-aperture method has short computation times associated with the coding of the tomographic information because of the applicability of fast fourier transform algorithms. However, because the information is collected in the frequency domain there does not appear to be a satisfactory approach to attenuation correction.
Pseudo random coded aperture imaging introduces the variable of time into a code pattern. By correlating the observed data with the known, time variant, characteristic of the collimator it is possible to reduce the data collected to a set of pinhole images. As such, the preprocessed data represents direct ray sums which can be reconstructed using conventional methods and attenuation corrections may be applied. The general method for time-modulated pseudorandoom coded aperture imaging is described by Koral K. F., Rogers W. L., and Knoll A. F., "Digital Tomographic Imaging with Time-Modulated Pseudorandom Coded Aperture and Anger Camera", The University of Michigan, J. Nucl. Med. 16:402-413, 1975 (hereinafter "Koral"). The disadvantage of the time modulated coded aperture method is the large reconstruction time that is noted in the University of Michigan study. In order to keep the reconstruction tractable, it is necessary to use a limited number of apertures (essentially pinhole apertures). The effect of the reduced number of apertures is to limit the information sampling in the frequency domain.